This invention relates to an improved apparatus for sensing the height of the platform assembly of a turret stockpicker.
In a materials handling vehicle, such as a turret stockpicker, an operator's platform mounted on a telescoping mast may be raised, along with a load handling assembly supporting load carrying forks, from floor level to the maximum height of the mast. The platform is raised until it reaches the top of the outer section of the mast. At this time, staging occurs, that is, as the platform is raised further, the platform picks up the outer section of the mast which telescopes on the inner or main section until the assembly reaches its maximum height, in the order of 35 feet. Height sensing means are used to control and limit the platform raising operation and other functions of the vehicle.
The height sensor in common use are potentiometers whose shafts are rotated by cables connected between the moveable platform and the base structure. These devices are not completely accurate, however, because of cable stretch, mechanical wear, non-linearity in the potentiometer itself, and drift in the analog-to-digital circuits that convert the potentiometer output to a digital signal useable by a computer or digital controller.
It is important for several reasons that the height sensor to be as accurate as possible since the output of this device controls various functions, such as the speed of the vehicle at various platform heights, the maximum height of the platform, and the speed at which the platform may be lifted.
Digital encoders may provide improved accuracy over analog devices, however digital devices also have their own limitations. For example, unlike potentiometers, digital encoders are not absolute position devices, so if the equipment were turned off when the platform is in other than the fully down position, the height of the platform cannot be ascertained with confidence.